Techniques for determining orientation of a target using light polarization

ABSTRACT

A method is provided that transmits a beam of co-propagating, cross-polarized light to a target. The method receives return light reflected from the target, which includes a first polarization and a second polarization. The method splits the return light into a first output corresponding to the first polarization and a second output corresponding to the second polarization using a first beam splitter. The method directs the first output to a first detector and directs the second output to a second detector. The method generates, by the first detector, a first electrical signal corresponding to the first polarization, and generates, by the second detector, a second electrical signal corresponding to the second polarization. The method determines an orientation of the target based on the first electrical signal and the second electrical signal, and generates a point cloud based on the orientation of the target.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/712,749 filed on Dec. 12, 2019, entitled “DETERMINING CHARACTERISTICSOF A TARGET USING POLARIZATION ENCODED COHERENT LIDAR,” the entirecontents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates generally to a frequency-modulatedcontinuous wave (FMCW) light detection and ranging (LIDAR) system thatutilizes polarization.

BACKGROUND

Traditional LIDAR systems operate by sending pulses toward a target andmeasuring the time the pulses take to reach the target and return. Insuch time-of-flight systems, the user learns information about thedistance to the object, which when coupled with a scanner can provide a3-D point cloud of the sensor's field-of-view.

SUMMARY

The present disclosure includes, without limitation, the followingexample implementations.

Some example implementations provide a method of operating a lightdetection and ranging (LIDAR) system. The method includes generating abeam of co-propagating, cross-polarized light using a first polarizingbeam splitter; and determining at least one of a material characteristicor orientation of a target using the co-propagating, cross-polarizedlight. In some embodiments, the method also includes splitting lightreflected from the target into a first output directed to a firstdetector and a second output directed to a second detector using asecond polarizing beam splitter; and determining a reflectivity and/ororientation of the target based on a comparison of a signal-to-noiseratio between signals from the first detector and the second detector.In some embodiments, the method also includes splitting theco-propagating, cross-polarized light into a local oscillator path and atarget path using a first beam splitter; receiving the local oscillatorpath light at a third polarizing beam splitter; transmitting the targetpath light to the target and directing light reflected from the targetto the second polarizing beam splitter using an optical pathdiscriminator; mixing the transmitted output of the third polarizingbeam splitter and the transmitted output of the second polarizing beamsplitter using a first light mixer; mixing the reflected output of thethird polarizing beam splitter and the reflected output of the secondpolarizing beam splitter using a second light mixer; receiving combinedlight from the first light mixer at the first detector; and receivingcombined light from the second light mixer at the second detector. Insome embodiments, the method also includes biasing light to the firstand second detectors in favor of light from the target path using thefirst and second light mixers. In some embodiments, the method alsoincludes receiving light from a plurality of laser sources at the firstpolarizing beam splitter. In some embodiments, the method also includesreceiving a transmitted output from the second polarizing beam splitterand a reflected output from the third polarizing beam splitter at thefirst light mixer; and receiving a reflected output from the secondpolarizing beam splitter and a transmitted output from the thirdpolarizing beam splitter at the second light mixer. In some embodiments,the method also includes transmitting light having different frequencypatterns using the plurality of laser sources. In some embodiments, themethod also includes combining a first pair of light inputs havingdissimilar wavelengths and opposite polarizations using a firstwavelength division multiplexer (WDM); combining a second pair of lightinputs having dissimilar wavelengths and opposite polarizations using asecond WDM; receiving combined light from the first WDM and the secondWDM at the first polarizing beam splitter; and separating the combinedlight from the first light mixer and the second light mixer bywavelength before directing the light to the first detector and thesecond detector. In some embodiments, the method also includes receivinglight at the first polarizing beam splitter from a second beam splitteroperatively coupled to a single laser source; receiving reflectedoutputs from the second and third polarizing beam splitters at the firstlight mixer; and receiving transmitted outputs from the second and thirdpolarizing beam splitters at the second light mixer.

Another example implementation provides a light detection and ranging(LIDAR) apparatus including a first polarizing beam splitter configuredto receive light at a first light input and a second light input andtransmit co-propagating, cross-polarized light to a target; a secondpolarizing beam splitter configured to split light reflected from thetarget into a transmitted output directed to a first detector and atransmitted output directed to a second detector; and a processingdevice configured to determine at least one of a material characteristicor an orientation of the target based on a comparison of asignal-to-noise ratio between signals from the first detector and thesecond detector. In some embodiments, the first detector and the seconddetector are configured to transmit signals to the processing device. Insome embodiments, the apparatus also includes a first beam splitterconfigured to split the co-propagating, cross-polarized light into alocal oscillator path and a target path; a third polarizing beamsplitter configured to receive the local oscillator path light; anoptical path discriminator configured to transmit the target path lightto a target and direct light reflected from the target to the secondpolarizing beam splitter; a first light mixer configured to mix lightfrom the transmitted output of the second polarizing beam splitter and areflected output of the third polarizing beam splitter; and a secondlight mixer configured to mix light from the reflected output of thesecond polarizing beam splitter and a transmitted output of the thirdpolarizing beam splitter, wherein the first detector is configured toreceive combined light from the first light mixer and the seconddetector is configured to receive combined light from the second lightmixer. In some embodiments, the first and second light mixers areconfigured to bias light to the first and second detectors in favor oflight from the target path. In some embodiments, the first light inputand the second light input are configured to receive light from aplurality of light sources. In some embodiments, the plurality of lightsources are configured to transmit light having different frequencypatterns. In some embodiments, the apparatus also includes a wavelengthdivision multiplexer (WDM) configured to combine light from the firstlaser source and the second laser source when they have dissimilarwavelengths. In some embodiments, the first and second light inputs areconfigured to receive light from a second beam splitter operativelycoupled to a single laser source. In some embodiments, the first lightmixer is configured to receive reflected outputs from the second andthird polarizing beam splitters, and the second light mixer isconfigured to receive transmitted outputs from the second and thirdpolarizing beam splitters. In some embodiments, the apparatus alsoincludes a second beam splitter configured to split the co-propagating,cross-polarized light into a second local oscillator path; a third beamsplitter configured to split a portion of the target path into a secondtarget path; a fourth polarizing beam splitter configured to receive thelocal oscillator path light; a second optical path discriminatorconfigured to transmit the second target path light to the target anddirect light reflected from the target to a fifth polarizing beamsplitter, the fifth polarizing beam splitter configured to split lightreflected from the target into a transmitted output directed to a thirddetector and a transmitted output directed to a fourth detector; a thirdlight mixer configured to mix light from the transmitted output of thefifth polarizing beam splitter and a reflected output of the fourthpolarizing beam splitter; and a fourth light mixer configured to mixlight from the reflected output of the fifth polarizing beam splitterand a transmitted output of the fourth polarizing beam splitter, whereinthe third detector is configured to receive combined light from thethird light mixer and the fourth detector is configured to receivecombined light from the fourth light mixer; wherein the processingdevice is further configured to determine at least one of a materialcharacteristic or an orientation of the target based on a comparison ofa signal-to-noise ratio between signals from the first detector, thesecond detector, the third detector, and the fourth detector. In someembodiments, the apparatus also includes at least one variablepolarization rotator located after the first and second beam splittersand configured to transform a polarization state of the polarized lightdirected to the target.

Another example implementation provides a light detection and ranging(LIDAR) system comprising: a plurality of optical circuits to generateand receive a laser beam, the plurality of optical circuits comprising:a first polarizing beam splitter configured to transmit co-propagating,cross-polarized light to a target; and a second polarizing beam splitterconfigured to split light reflected from the target into a first outputdirected to a first detector and a second output directed to a seconddetector; and a processing device operatively coupled with the pluralityof optical circuits to compare a signal-to-noise ratio from the firstand second detectors and determine a material characteristic and/ororientation of the target based on the comparison.

These and other features, aspects, and advantages of the presentdisclosure will be apparent from a reading of the following detaileddescription together with the accompanying figures, which are brieflydescribed below. The present disclosure includes any combination of two,three, four or more features or elements set forth in this disclosure,regardless of whether such features or elements are expressly combinedor otherwise recited in a specific example implementation describedherein. This disclosure is intended to be read holistically such thatany separable features or elements of the disclosure, in any of itsaspects and example implementations, should be viewed as combinableunless the context of the disclosure clearly dictates otherwise.

It will therefore be appreciated that this Brief Summary is providedmerely for purposes of summarizing some example implementations so as toprovide a basic understanding of some aspects of the disclosure.Accordingly, it will be appreciated that the above described exampleimplementations are merely examples and should not be construed tonarrow the scope or spirit of the disclosure in any way. Other exampleimplementations, aspects, and advantages will become apparent from thefollowing detailed description taken in conjunction with theaccompanying figures which illustrate, by way of example, the principlesof some described example implementations.

BRIEF DESCRIPTION OF THE FIGURES

Embodiments and implementations of the present disclosure will beunderstood more fully from the detailed description given below and fromthe accompanying drawings of various aspects and implementations of thedisclosure, which, however, should not be taken to limit the disclosureto the specific embodiments or implementations, but are for explanationand understanding only.

FIG. 1 illustrates a LIDAR system according to example implementationsof the present disclosure.

FIG. 2 illustrates a LIDAR system for performing material estimationaccording to example implementations of the present disclosure.

FIG. 3A illustrates a triangle wave frequency modulation and echosignal, according to an example embodiment of the present disclosure.

FIG. 3B illustrates a triangle wave frequency modulation and echosignal, as well as a counter-chirped signal, according to an exampleembodiment of the present disclosure.

FIG. 4 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure.

FIG. 5 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure.

FIG. 6 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure.

FIG. 7 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure.

FIG. 8 illustrates a LIDAR system for performing speckle reduction ormaterial estimation according to example implementations of the presentdisclosure.

FIG. 9 illustrates another LIDAR system for performing speckle reductionor material estimation according to example implementations of thepresent disclosure.

FIG. 10 illustrates another LIDAR system for performing specklereduction or material estimation according to example implementations ofthe present disclosure.

FIG. 11 illustrates a LIDAR system for performing speckle reduction andmaterial estimation according to example implementations of the presentdisclosure.

FIG. 12 illustrates a LIDAR system for performing speckle reduction ormaterial estimation according to example implementations of the presentdisclosure.

FIG. 13 depicts a flow diagram of a method for performing materialestimation using polarized light in accordance with exampleimplementations of the present disclosure.

FIG. 14 depicts a flow diagram of another method for performing materialestimation using polarized light in accordance with exampleimplementations of the present disclosure.

FIG. 15 depicts a flow diagram of a method for performing specklereduction using polarized light in accordance with exampleimplementations of the present disclosure.

FIG. 16 depicts a flow diagram of another method for performing specklereduction using polarized light in accordance with exampleimplementations of the present disclosure.

FIG. 17 is a block diagram of an example apparatus that may perform oneor more of the operations described herein, in accordance with someembodiments.

DETAILED DESCRIPTION

Example implementations of the present disclosure are directed to animproved scanning LIDAR system. Example implementations of the presentdisclosure are based on a type of LIDAR that uses polarization in orderto gather additional data based on the different ways polarized lightreflects off materials. Historically, LIDAR systems have not used lightpolarization as a way to gather additional data about a target or anenvironment; thus, such systems have not taken advantage of all theinformation that can potentially be gained from light received using aLIDAR system.

Traditional LIDAR systems operate by sending pulses toward a target andmeasuring the time the pulses take to reach the target and return. Insuch time-of-flight systems, the user learns information about thedistance to the object, which when coupled with a scanner can provide a3-D point cloud of the sensor's field-of-view. However, thesetraditional systems may have certain limitations, e.g., the inability todirectly measure the velocity of the target, and the susceptibility tocross-talk from other like systems. An alternative LIDAR system canemploy frequency-modulated continuous wave (FMCW) techniques to measuredistance and velocity to yield a 4-D LIDAR system.

Example embodiments of the present disclosure additionally usepolarization to enhance system performance by enabling orientationestimation and reducing signal-to-noise degradation due to speckleeffects. Such systems can provide depth and velocity informationsimultaneously for each location across a 2-D scan pattern. By usingpolarization optics, one can ascertain further information about atarget's orientation. Furthermore, employing different configurations orpolarization optics can mitigate the deleterious effects of speckle onthe signal-to-noise ratio (SNR) of the system.

Example embodiments of the present disclosure involve using aco-propagating, cross-polarized beam of light as an outgoing signal. Dueto polarization-based differences in the reflectivities of targets,these two signals can have different SNR measurements. One can then usethis information to provide further insights into the surroundingenvironment including, but not limited to, determining materialreflectivity or object orientation.

Employing similar system components and geometries, but operated withdifferent polarization properties, one can mitigate the harmful effectsof speckle on the SNR. Speckle can occur, for example, due tophase-front variations in turbulent air, or wavelength-scale reflectionsfrom uneven surfaces. These effects contribute to SNR fluctuations onthe signal if the outgoing signal is fixed. By rapidly modulating thepolarization state of the signal transmitted to a target, one canaverage the spatial-mode coherence, thereby reducing the negativeeffects of speckle.

Example implementations of the present disclosure can provideenhancements to any sensing market, such as, but not limited to,transportation, manufacturing, metrology, medical, and security. Forexample, in the automotive industry, such a device can assist withspatial awareness for automated driver assist systems or self-drivingvehicles.

FIG. 1 illustrates a LIDAR system 100 according to exampleimplementations of the present disclosure. The LIDAR system 100 includesone or more of each of a number of components, but may include fewer oradditional components than shown in FIG. 1 . As shown, the LIDAR system100 includes optical circuits 101. The optical circuits 101 may includea combination of active optical components and passive opticalcomponents. Active optical components may generate, amplify, or detectoptical signals and the like. In some examples, the active opticalcircuit includes lasers at different wavelengths, one or more opticalamplifiers, one or more optical detectors, or the like.

Passive optical circuits may include one or more optical fibers orwaveguides to carry optical signals, and route and manipulate opticalsignals to appropriate input/output ports of the active optical circuit.The passive optical circuits may also include one or more fibercomponents such as taps, wavelength division multiplexers,splitters/combiners, polarization beam splitters, collimators,circulators, isolators, or the like. In some embodiments, as discussedfurther below, the passive optical circuits may include components totransform the polarization state and direct received polarized light tooptical detectors using a PBS.

An optical scanner 102 includes one or more scanning mirrors that areswept along respective orthogonal axes to steer optical signals to scanan environment according to a scanning pattern. For instance, thescanning mirrors may be rotatable by one or more galvanometers. Theoptical scanner 102 also collects light incident upon any objects in theenvironment into a return laser beam that is returned to the passiveoptical circuit component of the optical circuits 101. For example, thereturn laser beam may be directed to an optical detector by apolarization beam splitter. In addition to the mirrors andgalvanometers, the optical scanning system may include components suchas a quarter-wave plate, lens, anti-reflective coated window or thelike.

To control and support the optical circuits 101 and optical scanner 102,the LIDAR system 100 includes LIDAR control systems 110. The LIDARcontrol systems 110 may include a processing device for the LIDAR system100. In embodiments, the processing device may be one or moregeneral-purpose processing devices such as a microprocessor, centralprocessing unit, or the like. More particularly, the processing devicemay be a complex instruction set computing (CISC) microprocessor,reduced instruction set computer (RISC) microprocessor, very longinstruction word (VLIW) microprocessor, or processor implementing otherinstruction sets, or processors implementing a combination ofinstruction sets. The processing device may also be one or morespecial-purpose processing devices such as an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA), adigital signal processor (DSP), network processor, or the like.

In some embodiments, the LIDAR control systems 110 may include a signalprocessing unit 112 such as a digital signal processor. The LIDARcontrol systems 110 are configured to output digital control signals tocontrol optical drivers 103. In some embodiments, the digital controlsignals may be converted to analog signals through signal conversionunit 106. For example, the signal conversion unit 106 may include adigital-to-analog converter. The optical drivers 103 may then providedrive signals to active components of optical circuits 101 to driveoptical sources such as lasers and amplifiers. In some embodiments,several optical drivers 103 and signal conversion units 106 may beprovided to drive multiple optical sources.

The LIDAR control systems 110 are also configured to output digitalcontrol signals for the optical scanner 102. A motion control system 105may control the optical scanner 102 based on control signals receivedfrom the LIDAR control systems 110. For example, a digital-to-analogconverter may convert coordinate routing information from the LIDARcontrol systems 110 to signals interpretable by the galvanometers in theoptical scanner 102. In some embodiments, a motion control system 105may also return information to the LIDAR control systems 110 about theposition or operation of components of the optical scanner 102. Forexample, an analog-to-digital converter may in turn convert informationabout the galvanometers' position to a signal interpretable by the LIDARcontrol systems 110.

The LIDAR control systems 110 are further configured to analyze incomingdigital signals. In this regard, the LIDAR system 100 includes opticalreceivers 104 to measure one or more beams received by optical circuits101. For example, a reference beam receiver may measure the amplitude ofa reference beam from the active optical circuit, and ananalog-to-digital converter converts signals from the reference receiverto signals interpretable by the LIDAR control systems 110. The opticalreceivers 104 may be in communication with a signal conditioning unit107, in some embodiments. Target receivers measure the optical signalthat carries information about the range and velocity of a target in theform of a beat frequency, modulated optical signal. The reflected beammay be mixed with a second signal from a local oscillator. The opticalreceivers 104 may include a high-speed analog-to-digital converter toconvert signals from the target receiver to signals interpretable by theLIDAR control systems 110.

In some applications, the LIDAR system 100 may additionally include oneor more imaging devices 108 configured to capture images of theenvironment, a global positioning system 109 configured to provide ageographic location of the system, or other sensor inputs. The LIDARsystem 100 may also include an image processing system 114. The imageprocessing system 114 can be configured to receive the images andgeographic location, and send the images and location or informationrelated thereto to the LIDAR control systems 110 or other systemsconnected to the LIDAR system 100.

In operation according to some examples, the LIDAR system 100 isconfigured to use nondegenerate laser sources to simultaneously measurerange and velocity across two dimensions. This capability allows forreal-time, long range measurements of range, velocity, azimuth, andelevation of the surrounding environment. In some exampleimplementations, the system points multiple modulated laser beams to thesame target.

In some examples, the scanning process begins with the optical drivers103 and LIDAR control systems 110. The LIDAR control systems 110instruct the optical drivers 103 to independently modulate one or morelasers, and these modulated signals propagate through the passiveoptical circuit to the collimator. The collimator directs the light atthe optical scanning system that scans the environment over apreprogrammed pattern defined by the motion control subsystem. Theoptical circuits may also include polarization wave plates to transformthe polarization of the light as it leaves the optical circuits 101. Inembodiments, the polarization wave plate may be a quarter-wave plateand/or a half-wave plate. A portion of the polarized light may also bereflected back to the optical circuits 101. For example, lensing orcollimating systems may have natural reflective properties or areflective coating to reflect a portion of the light back to the opticalcircuits 101.

Optical signals reflected back from the environment pass through theoptical circuits 101 to the receivers. Because the polarization of thelight has been transformed, it may be reflected by a polarization beamsplitter along with the portion of polarized light that was reflectedback to the optical circuits 101. Accordingly, rather than returning tothe same fiber or waveguide as an optical source, the reflected light isreflected to separate optical receivers. These signals interfere withone another and generate a combined signal. Each beam signal thatreturns from the target produces a time-shifted waveform. The temporalphase difference between the two waveforms generates a beat frequencymeasured on the optical receivers (photodetectors). The combined signalcan then be reflected to the optical receivers 104. Configuration ofoptical circuits 101 for polarizing and directing beams to the opticalreceivers 104 are described further below.

The analog signals from the optical receivers 104 are converted todigital signals using ADCs. The digital signals are then sent to theLIDAR control systems 110. A signal processing unit 112 may then receivethe digital signals and interpret them. In some embodiments, the signalprocessing unit 112 also receives position data from the motion controlsystem 105 and optical scanner 102, as well as image data from the imageprocessing system 114. The signal processing unit 112 can then generatea 3D point cloud with information about range and velocity of points inthe environment as the optical scanner 102 scans additional points. Thesignal processing unit 112 can also overlay a 3D point cloud data withthe image data to determine velocity and distance of objects in thesurrounding area. The system also processes the satellite-basednavigation location data to provide a precise global location.

FIG. 2 illustrates a LIDAR system for performing material estimationaccording to example implementations of the present disclosure. TheLIDAR system includes one or more of each of a number of components, butmay include fewer or additional components than shown in FIG. 2 . Asshown, the LIDAR system includes a laser source 201, a first beamsplitter 203, and a first polarizing beam splitter 205. In this exampleembodiment, a single laser source 201 is split using the first beamsplitter 203, and then recombined using the first polarizing beamsplitter 205 to generate a beam of co-propagating, cross-polarizedlight. The beam of cross-polarized light is then split into a targetpath and a local oscillator (LO) path using a second beam splitter 207.The target path light can be amplified using an optical amplifier 209,and directed to a target 217 through an optical path discriminator 211.The target path light can be directed to the target 217 through a lenssystem 213 and a polarization wave plate 215, in some embodiments. Thelight reflected from the target 217 can then be directed through theoptical path discriminator 211 to a second polarizing beam splitter 219.According to some embodiments, the optical path discriminator 211 can bea circulator or a beam splitter. In this example embodiment, the LO pathlight is transmitted from the second beam splitter 207 to a thirdpolarizing beam splitter 229. The second polarizing beam splitter 219and the third polarizing beam splitter 229 are both configured totransmit light to the first mixer 221 and the second mixer 225. In thisexample embodiment, the first mixer 221 and second mixer 225 aresingle-ended and each connects to the first detector 223 and the seconddetector 227, respectively. However, the first and second mixers 221,225 could have two or more outputs according to other embodiments.

According to the embodiment shown in FIG. 2 , the reflected outputs fromeach of the second polarizing beam splitter 219 and the secondpolarizing beam splitter 229 are both directed to the first mixer 221,while the transmitted outputs from the second and third polarizing beamsplitters 219, 229 are both directed to the second mixer 225. The use ofthe second polarizing beam splitter 219 and the second polarizing beamsplitter 229 can differentiate the SNR measured on the first detector223 and the second detector 227. In some embodiments, the first andsecond mixers 221, 225 do not mix light equally from the LO path and thetarget path, but instead bias light coming from the target path. Forexample, the first and second mixers 221, 225 can bias light output tothe first and second detectors 223, 227 at a ratio of 80/20, 90/10, or99/1 in favor of the target path light, in some embodiments.

In this example embodiment, the system also includes a signal processingunit 250 in communication with the first detector 223 and the seconddetector 227 and configured to analyze signals from the detectors. Thedetectors measure the optical signal from the mixers and generateproportional electric signals. The signal processing unit 250 cancompare the intensities of the signals from the first detector 223 andthe second detector 227 and provide insights into the target materialreflectivity, surface quality, or orientation. For example, an icepatch, puddle of water, or other reflective surface can largely reflectlight only in one polarization parallel to the horizon, such that thereis a large amount of light detected polarized parallel to the horizonbut not much light detected polarized perpendicular to the horizon. Ifthis is detected, the signal processing unit 250 can determine thereflectivity and orientation of the target material, and can potentiallyknow that there is a puddle or ice patch ahead. A similar phenomenon canalso occur with windows on a building that are highly reflective in onepolarization.

The input ports of the first polarizing beam splitter 205 areco-polarized, in this example embodiment, but the splitter performs thepolarization rotation upon reflection to create a beam ofco-propagating, cross-polarized light. When used in reverse, as thesecond polarizing beam splitter 219 and third polarizing beam splitter229 are arranged, a cross-polarized source will filter the twopolarizations to their respective output ports. This behavior is not thesame if free-space polarizing beam splitters are used, wherein apolarization rotator (such as a half wave-plate) is used on at least oneof the ports in order to co-polarize the beams.

FIG. 3A illustrates a triangle wave frequency modulation and echosignal, according to an example embodiment of the present disclosure.The FMCW LIDAR systems described in this disclosure modulate thefrequency of the laser with a center frequency f_(c), over a sweepduration T_(s). The modulation can take any number of possible sweeppatterns, such as a sawtooth, or a triangle wave as shown in FIG. 3 ,which also shows a time-shifted “echo” (signal returned from a target)using the dashed line and delayed by Δt. The echo has a frequency shiftdue to the Doppler effect of a moving object. In this case, the Dopplershift Δf_(doppler) and the echo create different beat tones on theup-sweep and down-sweep of the triangle wave modulation. These beatfrequencies are labeled Δ_(fup) and Δ_(fdn), respectively. The range andvelocity can be calculated directly from these parameters usingequations (1) and (2) below.

$\begin{matrix}{R = {\frac{cT_{s}}{4B_{s}}( {{\Delta f_{up}} + {\Delta f_{dn}}} )}} & (1)\end{matrix}$ $\begin{matrix}{V = {\frac{\lambda_{c}}{4}( {{\Delta f_{dn}} - {\Delta f_{up}}} )}} & (2)\end{matrix}$

In equations (1) and (2) above, Bs is the sweep bandwidth for themodulated signal, and λ_(c) is the center wavelength of that sweep(defined by c/f_(C)).

FIG. 3B illustrates a triangle wave frequency modulation and echosignal, as well as a counter-chirped signal, according to an exampleembodiment of the present disclosure. According to some embodiments ofthe present disclosure, two laser sources can be used and one canmodulate the two lasers (potentially at different wavelengths) withdifferent patterns, e.g., the counter-chirp triangle modulation shown inFIG. 3B. In this example, the transmitted lasers are shown as lines 301and 303, and their respective echoes are shown as lines 305 and 307.This enables real-time range and velocity measurements since Δ_(fup) andΔ_(fdn) can be retrieved simultaneously, although reduced SNR in onepolarization due to the various material properties of targets canaffect measurements. Potential solutions to remedy this problem arediscussed later in this disclosure. Although triangle waves have beenused in the examples shown in FIGS. 3A and 3B, this application is notlimited to such waves, and the frequency modulation pattern for anylaser can be different from the other lasers.

FIG. 4 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure. The LIDAR system includes one or more of each of a number ofcomponents, but may include fewer or additional components than shown inFIG. 4 . As shown, the LIDAR system includes two laser sources 401, 402and a first polarizing beam splitter 405. In this example embodiment,the first laser source 401 and the second laser source 402 are bothconnected to inputs of the first polarizing beam splitter 405 togenerate a beam of co-propagating, cross-polarized light. While thisexample illustrates an embodiment with two laser sources, theapplication is not limited to two laser sources, and a larger number oflight sources can be used in various embodiments. The beam ofcross-polarized light is then split into a target path and a localoscillator (LO) path using a beam splitter 407. The target path lightcan be amplified using an optical amplifier 409, and directed to atarget 417 through an optical path discriminator 411. The target pathlight can be directed to the target 417 through a lens system 413 and apolarization wave plate 415, in some embodiments. The light reflectedfrom the target 417 can then be directed through the optical pathdiscriminator 411 to a second polarizing beam splitter 419. According tosome embodiments, the optical path discriminator 411 can be a circulatoror a beam splitter. In this example embodiment, the LO path light istransmitted from the beam splitter 407 to a third polarizing beamsplitter 429. The second polarizing beam splitter 419 and the thirdpolarizing beam splitter 429 are both configured to transmit light tothe first mixer 421 and the second mixer 425.

In this example embodiment, two laser sources 401, 402 are used, andtherefore the appropriate polarization should be routed to the finalmixers 421, 425 in order to maximize the mixing efficiency. Becausedissimilar lasers do not mix well, note that the beams that are mixed atthe first mixer 421 and the second mixer 425 are from opposite ports ofthe second polarizing beam splitter 419 and the third polarizing beamsplitter 429. This is due to the s-polarization to p-polarizationrotation process (and vice versa) that occurs when passing a linearpolarization through the polarization wave plate 415, off a target, andback through the polarization wave plate 415.

In this example embodiment, the first mixer 421 and second mixer 425 aresingle-ended and each connects to the first detector 423 and the seconddetector 427, respectively. However, the first and second mixers 421,425 could have two or more outputs according to other embodiments. Insome embodiments, the first and second mixers 421, 425 do not mix lightequally from the LO path and the target path, but instead bias lightcoming from the target path. For example, the first and second mixers421, 425 can bias light output to the first and second detectors 423,427 at a ratio of 80/20, 90/10, or 99/1 in favor of the target pathlight, in some embodiments. This biasing of the light from the targetpath can increase the detection of the target path light, which mayresult in a more accurate detection of the target.

FIG. 5 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure. The LIDAR system includes one or more of each of a number ofcomponents, but may include fewer or additional components than shown inFIG. 5 . Similar to the embodiment shown above in FIG. 4 , the LIDARsystem includes two laser sources 501, 502 and a first polarizing beamsplitter 505. In this example embodiment, the first laser source 501 andthe second laser source 502 are both connected to inputs of the firstpolarizing beam splitter 505 to generate a beam of co-propagating,cross-polarized light. In this example embodiment, a supplementaloptical circuit is included that can be used to create a signalreference. This reference circuit includes a first beam splitter 506that can split the cross-polarized beam of light and direct a portion ofthe light to a second splitter 531, an optical delay line 535, a thirdbeam splitter 533, a fourth polarizing beam splitter 537, and tworeference detectors 538, 539. This reference signal can be tracked usingthe reference detectors 538, 539 and subsequently used to compensate forperturbations in the laser sources and to calibrate the system'sperformance. This reference circuit can be added to any of the otherembodiments described in this disclosure.

The beam of cross-polarized light is also split into a target path and alocal oscillator (LO) path using a fourth beam splitter 507. The targetpath light can be amplified using an optical amplifier 509, and directedto a target 517 through an optical path discriminator 511. The targetpath light can be directed to the target 517 through a lens system 513and a polarization wave plate 515, in some embodiments. The lightreflected from the target 517 can then be directed through the opticalpath discriminator 511 to a second polarizing beam splitter 519.According to some embodiments, the optical path discriminator 511 can bea circulator or a beam splitter. In this example embodiment, the LO pathlight is transmitted from the fourth beam splitter 507 to a thirdpolarizing beam splitter 529. The second polarizing beam splitter 519and the third polarizing beam splitter 529 are both configured totransmit light to the first mixer 521 and the second mixer 525.

In this example embodiment, two laser sources 501, 502 are used, andtherefore the appropriate polarization should be routed to the finalmixers 521, 525 in order to maximize the mixing efficiency. Becausedissimilar lasers do not mix well, the beams that are mixed at the firstmixer 521 and the second mixer 525 are from opposite ports of the secondpolarizing beam splitter 519 and the third polarizing beam splitter 529.This is due to the s-polarization to p-polarization rotation process(and vice versa) that occurs when passing a linear polarization throughthe polarization wave plate 515, off a target 517, and back through thepolarization wave plate 515.

In this example embodiment, the first mixer 521 and second mixer 525 aresingle-ended and each connects to the first detector 523 and the seconddetector 527, respectively. However, the first and second mixers 521,525 could have two or more outputs according to other embodiments. Insome embodiments, the first and second mixers 521, 525 do not mix lightequally from the LO path and the target path, but instead bias lightcoming from the target path. For example, the first and second mixers521, 525 can bias light output to the first and second detectors 523,527 at a ratio of 80/20, 90/10, or 99/1 in favor of the target pathlight, in some embodiments. This biasing of the light from the targetpath can increase the detection of the target path light, which mayresult in a more accurate detection of the target.

FIG. 6 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure. The LIDAR system includes one or more of each of a number ofcomponents, but may include fewer or additional components than shown inFIG. 6 . This example embodiment shows a two-laser design of apolarization-enhanced LIDAR system where two beams are sent to a target617. For simplicity, only two beams are shown in this example, but thereare fundamentally no restrictions to the number of beams that can besent to a target, and the system can be scaled up or down according toeach embodiment. As shown, the LIDAR system includes two laser sources601, 602 and a first polarizing beam splitter 605. In this exampleembodiment, the first laser source 601 and the second laser source 602are both connected to inputs of the first polarizing beam splitter 605to generate a beam of co-propagating, cross-polarized light. Increasingthe number of beams requires increasing the number of splitters used, orthe number of outputs from each splitter that bookend the opticalamplifier 609.

In this example embodiment, the beam of cross-polarized light is splitinto two LO paths using a first splitter 606 and a second splitter 607ahead of the optical amplifier 609. In an alternative embodiment, thefirst and second splitters 606, 607 can be combined into a differenttype of splitter, such as a 1×3 splitter, or an active switch in someembodiments. This substitution or combination can simplify the system.The target path light is directed from the optical amplifier 609 to athird splitter 630, which divides the light into two target paths. Eachtarget path can be directed to the target 617 through a first opticalpath discriminator 611 or a second optical path discriminator 631. Thefirst target path light can be directed to the target 617 through afirst lens system 613 and a first polarization wave plate 615, while thesecond target path light is directed to through a second lens system 633and a second polarization wave plate 635. The first beam of lightreflected from the target 617 can then be directed through the firstoptical path discriminator 611 to a second polarizing beam splitter 619,while the second beam of light reflected from the target 617 can bedirected through the second optical path discriminator 631 to a fourthpolarizing beam splitter 639. According to some embodiments, the opticalpath discriminators 611, 631 can be circulators or beam splitters.

In this example embodiment, the first LO path light is transmitted fromthe first beam splitter 606 to a fifth polarizing beam splitter 649,while the second LO path light is transmitted from the second beamsplitter 607 to a third polarizing beam splitter 629. The secondpolarizing beam splitter 619 and the third polarizing beam splitter 629are both configured to transmit light to the first mixer 621 and thesecond mixer 625. Similarly, the fourth polarizing beam splitter 639 andthe fifth polarizing beam splitter 649 are both configured to transmitlight to the third mixer 645 and the fourth mixer 641.

In this example embodiment, two laser sources 601, 602 are used, andtherefore the appropriate polarization should be routed to the finalmixers 621, 625, 641, 645 in order to maximize the mixing efficiency.Because dissimilar lasers do not mix well, note that the beams that aremixed at the first mixer 621 and the second mixer 625 are from oppositeports of the second polarizing beam splitter 619 and the thirdpolarizing beam splitter 629. Similarly, the beams mixed at the thirdmixer 641 and the fourth mixer 645 are from opposite ports of the fourthpolarizing beam splitter 639 and the fifth polarizing beam splitter 649.This is due to the s-polarization to p-polarization rotation process(and vice versa) that occurs when passing a linear polarization throughthe polarization wave plates 615, 635 off a target 617, and back throughthe polarization wave plates 615, 635. In this example embodiment, thefirst mixer 621, second mixer 625, third mixer 641, and fourth mixer 645are single-ended and each connects to the first detector 623, seconddetector 627, third detector 643, and fourth detector 647, respectively.

FIG. 7 illustrates another LIDAR system for performing materialestimation according to example implementations of the presentdisclosure. Specifically, FIG. 7 shows an example of a multiple lasermulti-beam embodiment of a polarization enhanced LIDAR system. The LIDARsystem includes one or more of each of a number of components, but mayinclude fewer or additional components than shown in FIG. 7 .

As shown, the LIDAR system includes four laser sources 761, 763, 701,702 and first and second wavelength division multiplexers (WDM) 765,760. The system uses two wavelengths for each polarization, with lasers761 and 763 having p-polarization and lasers 701 and 702 havings-polarization. In this fashion, one can counter-chirp the two lasers toobtain real-time range and velocity measurements in addition toreal-time material estimation information due to the variable responsein polarization. One skilled in the art will realize that WDMs are notrequired to combine lasers into a single spatial mode if another opticaldegree of freedom is used for distinguishing between the lasers.Although two WDMs are shown in this example, the present disclosure isnot limited to two colors. Any number of colors can be used in thevarious embodiments of this disclosure.

In this example embodiment the first and second WDMs are connected tothe inputs of a first polarizing beam splitter 705 to generate theinitial beam, and this beam is split into two LO paths using a firstsplitter 706 and a second splitter 707 ahead of the optical amplifier709. In an alternative embodiment, the first and second splitters 706,707 can be combined into a different type of splitter, such as a 1×3splitter, or an active switch in some embodiments. The target path lightis directed from the optical amplifier 709 to a third splitter 730, thatdivides the light into two target paths. Each target path can bedirected to the target 717 through a first optical path discriminator711 or a second optical path discriminator 731. The first target pathlight can be directed to the target 717 through a first lens system 713and a first polarization wave plate 715, while the second target pathlight is directed to through a second lens system 733 and a secondpolarization wave plate 735. The first beam of light reflected from thetarget 717 can then be directed through the first optical pathdiscriminator 711 to a second polarizing beam splitter 719, while thesecond beam of light reflected from the target 717 can be directedthrough the second optical path discriminator 731 to a fourth polarizingbeam splitter 739. According to some embodiments, the optical pathdiscriminators 711, 731 can be circulators or beam splitters. In anadditional embodiment, a variable polarization rotator (VPR) can beintroduced after the second beam splitter 707. A single VPR (not shown)could be introduced before the third splitter 730, in some embodiments,while in other embodiments two (or more) VPRs may be introduced afterthe third splitter 730.

In this example embodiment, the first LO path light is transmitted fromthe first beam splitter 706 to a fifth polarizing beam splitter 749,while the second LO path light is transmitted from the second beamsplitter 707 to a third polarizing beam splitter 729. The secondpolarizing beam splitter 719 and the third polarizing beam splitter 729are both configured to transmit light to the first mixer 721 and thesecond mixer 725. Similarly, the fourth polarizing beam splitter 739 andthe fifth polarizing beam splitter 749 are both configured to transmitlight to the third mixer 741 and the fourth mixer 745.

In this example embodiment, because multiple laser sources are used theappropriate polarization should be routed to the final mixers 721, 725,741, 745 in order to maximize the mixing efficiency. Because dissimilarlasers do not mix well, note that the beams that are mixed at the firstmixer 721 and the second mixer 725 are from opposite ports of the secondpolarizing beam splitter 719 and the third polarizing beam splitter 729.Similarly, the beams mixed at the third mixer 741 and the fourth mixer745 are from opposite ports of the fourth polarizing beam splitter 739and the fifth polarizing beam splitter 749. This is due to thes-polarization to p-polarization rotation process (and vice versa) thatoccurs when passing a linear polarization through the polarization waveplates 715, 735 off a target 717, and back through the polarization waveplates 715, 735. In this example embodiment, the first mixer 721 directsmixed light to a third WDM 723, which separates the light by wavelengthand directs it to the first and second detectors 751, 752. Likewise, thesecond mixer directs mixed light to a fourth WDM 727, which separatesthe light by wavelength and directs it to the third and fourth detectors753, 754; the third mixer 741 directs mixed light to a fifth WDM 743,which separates the light by wavelength and directs it to the fifth andsixth detectors 757, 758; and the fourth mixer 745 directs mixed lightto a sixth WDM 747, which separates the light by wavelength and directsit to the seventh and eighth detectors 755, 756.

FIG. 8 illustrates a LIDAR system for performing speckle reductionaccording to example implementations of the present disclosure. TheLIDAR system includes one or more of each of a number of components, butmay include fewer or additional components than shown in FIG. 8 . Thisexample embodiment shows an optical circuit that can remove the need fordual-axis components by using a single laser 801 and a polarizing beamsplitter 805 at the source, and adding a variable polarization rotator(VPR) 812 to achieve speckle reduction. The beam of polarized light isthen split into a target path and a local oscillator (LO) path using afirst beam splitter 807. The target path light can be amplified using anoptical amplifier 809, and directed to a target 817 through an opticalpath discriminator 811. The target path light can be directed to thetarget 817 through the variable polarization rotator 812, a lens system813, and a polarization wave plate 815, in some embodiments. The lightreflected from the target 817 can then be directed through the opticalpath discriminator 811 to a second polarizing beam splitter 819. In thisexample embodiment, the LO path light is transmitted from the first beamsplitter 807 to a second beam splitter 829. The second polarizing beamsplitter 819 and the second beam splitter 829 are both configured totransmit light to the first mixer 821 and the second mixer 825. In thisexample embodiment, the first mixer 821 and second mixer 825 aresingle-ended and each connects to the first detector 823 and the seconddetector 827, respectively. However, the first and second mixers 821,825 could have two or more outputs according to other embodiments.

The VPR 812 can be controlled, in some embodiments, using a polarizationrotating controller 850 that is configured to toggle the VPR 812 betweentwo settings: one setting that performs no rotation, allowingp-polarization to stay p-polarized; and a second setting that performs,for example, a 90° rotation converting a p-polarized light tos-polarization. With this circuit design, the second splitter 829 can bea 2×2 splitter rather than a polarizing beam splitter, since all of thelight propagating through it is co-polarized, and the second polarizingbeam splitter 819 converts the cross-polarized light from the opticalpath discriminator 811 to co-polarized light on both waveguides leadingto the mixers 821, 825 before the respective detectors 823, 827. Inexemplary embodiments, the polarization rotating controller 850 canachieve speckle reduction by controlling the operation of the VPR 812 totransform the polarization state of the target path light at a ratefaster than the rate of sampling by the detectors 823, 827.

According to various embodiments, the VPR 812 can be placed before orafter the optical path discriminator 811, although placement beforerequires a dual-axis optical path discriminator 811. Similarly, it canbe placed before the optical amplifier 809, but again it requires adual-axis amplifier which can lead to crosstalk and increased noise. Insome embodiments, the first polarizing beam splitter 805 can be omittedif the laser source 801 generates a polarized beam.

FIG. 9 illustrates another LIDAR system for performing speckle reductionaccording to example implementations of the present disclosure. TheLIDAR system includes one or more of each of a number of components, butmay include fewer or additional components than shown in FIG. 9 . Thisexample embodiment shows an optical circuit where the VPR 912 is movedahead of a first beam splitter 907 and reintroduces a third polarizingbeam splitter 929 for the LO path. The beam of polarized light generatedby the laser source 901 and the first polarizing beam splitter 905 issplit into a target path and a LO path using a first beam splitter 907.The target path light can be amplified using an optical amplifier 909,and directed to a target 917 through an optical path discriminator 911.The target path light can be directed to the target 917 through a lenssystem 913 and a polarization wave plate 915, in some embodiments. Thelight reflected from the target 917 can then be directed through theoptical path discriminator 911 to a second polarizing beam splitter 919.The second polarizing beam splitter 919 and the third polarizing beamsplitter 929 are both configured to transmit light to the first mixer921 and the second mixer 925. In this example embodiment, the firstmixer 921 and second mixer 925 are single-ended and each connects to thefirst detector 923 and the second detector 927, respectively. However,the first and second mixers 921, 925 could have two or more outputsaccording to other embodiments.

The VPR 912 can be controlled, in some embodiments, using a polarizationrotating controller 950 that is configured to toggle the VPR 912 betweentwo settings: one setting that performs no rotation, allowingp-polarization to stay p-polarized; and a second setting that performs,for example, a 90° rotation converting a p-polarized light tos-polarization. This design can potentially be very useful if one usesthe VPR 912 to rapidly scramble (i.e. at a rate faster than the rate ofsampling by the detectors 923, 927) between various orthogonal pairs ofpolarizations. The rapid “scrambling” of the polarization shouldmitigate SNR fluctuation due to speckle effects. Meanwhile, the use oforthogonal polarizations enables the material estimation.

FIG. 10 illustrates another LIDAR system for performing specklereduction according to example implementations of the presentdisclosure. The LIDAR system includes one or more of each of a number ofcomponents, but may include fewer or additional components than shown inFIG. 10 . This example embodiment shows an optical circuit that is atwo-laser variant of the embodiment described above in reference to FIG.8 . Specifically, the system includes a first laser source 1001 andsecond laser source 1002 that are connected to inputs of a WDM 1005 tocombine the two lasers if they have dissimilar wavelengths. If twosimilar wavelengths are used, one can use any modal combiner (such as a2×2 splitter) to spatially couple the two lasers. The beam of light fromthe WDM 1005 is split into a target path and a LO path using a firstbeam splitter 1007. The target path light can be amplified using anoptical amplifier 1009, and directed to a VPR 1012 ahead of an opticalpath discriminator 1011. The target path light can be directed to thetarget 1017 through a lens system 1013 and a polarization wave plate1015, in some embodiments, and the light reflected from the target 1017can then be directed through the optical path discriminator 1011 to apolarizing beam splitter 1019. In this example embodiment, the LO pathlight is transmitted from the first beam splitter 1007 to a second beamsplitter 1029. The polarizing beam splitter 1019 and the second beamsplitter 1029 are both configured to transmit light to the first mixer1021 and the second mixer 1025. In this example embodiment, the firstmixer 1021 and second mixer 1025 are single-ended and each connects tothe first detector 1023 and the second detector 1027, respectively. Inthis particular embodiment, the VPR 1012 is placed before the opticalpath discriminator 1011, and therefore a dual-axis optical pathdiscriminator 1011 is used. As discussed above in reference to FIG. 7 ,the use of a WDM 1005 allows a counter-chirp to be used with the twolasers to obtain real-time range and velocity measurements in thisparticular embodiment.

FIG. 11 illustrates a LIDAR system for performing material estimationand speckle reduction according to example implementations of thepresent disclosure. Specifically, FIG. 11 shows an example of amulti-beam VPR-based LIDAR system for material estimation and specklereduction. The LIDAR system includes one or more of each of a number ofcomponents, but may include fewer or additional components than shown inFIG. 11 . As shown, the LIDAR system includes two laser sources 1101 and1102 that are combined using a WDM 1105. The use of a WDM 1105 allows acounter-chirp to be used with the two lasers to obtain real-time rangeand velocity measurements in this particular embodiment. In analternative embodiment, the LIDAR system illustrated in FIG. 11 can beimplemented with four laser sources and a first and second WDM, similarto the embodiment described above in reference to FIG. 7 .

In this example embodiment, the beam from the WDM 1105 is split into twoLO paths using a first splitter 1106 and a second splitter 1107 ahead ofthe optical amplifier 1109. In an alternative embodiment, the first andsecond splitters 1106, 1107 can be combined into a different type ofsplitter, such as a 1×3 splitter, or an active switch in someembodiments. The target path light is directed from the opticalamplifier 1109 to a third splitter 1130, that divides the light into twotarget paths. Each target path can be directed to a respective VPR 1190,1192. After passing through the first VPR 1190 and the second VPR 1192,the target path light can be directed to the target 1117 through a firstoptical path discriminator 1111 or a second optical path discriminator1131. The first target path light can be directed to the target 1117through a first lens system 1113 and a first polarization wave plate1115, while the second target path light is directed through a secondlens system 1133 and a second polarization wave plate 1135. The firstbeam of light reflected from the target 1117 can then be directedthrough the first optical path discriminator 1111 to a first polarizingbeam splitter 1119, while the second beam of light reflected from thetarget 1117 can be directed through the second optical pathdiscriminator 1131 to a second polarizing beam splitter 1139. Accordingto some embodiments, the optical path discriminators 1111, 1131 can becirculators or beam splitters.

In this example embodiment, the first LO path light is transmitted fromthe first beam splitter 1106 to a fifth beam splitter 1149, while thesecond LO path light is transmitted from the second beam splitter 1107to a fourth beam splitter 1129. The first polarizing beam splitter 1119and the fourth beam splitter 1129 are both configured to transmit lightto the first mixer 1121 and the second mixer 1125. Similarly, the secondpolarizing beam splitter 1139 and the fifth beam splitter 1149 are bothconfigured to transmit light to the third mixer 1145 and the fourthmixer 1141.

In this example embodiment, the first mixer 1121 directs mixed light toa second WDM 1123, which separates the light by wavelength and directsit to the first and second detectors 1151, 1152. Likewise, the secondmixer 1125 directs mixed light to a third WDM 1127, which separates thelight by wavelength and directs it to the third and fourth detectors1153, 1154; the third mixer 1141 directs mixed light to a fourth WDM1143, which separates the light by wavelength and directs it to thefifth and sixth detectors 1157, 1158; and the fourth mixer 1145 directsmixed light to a fifth WDM 1147, which separates the light by wavelengthand directs it to the seventh and eighth detectors 1155, 1156.

FIG. 12 illustrates a LIDAR system according to example implementationsof the present disclosure. The LIDAR system includes one or more of eachof a number of components, but may include fewer or additionalcomponents than shown in FIG. 12 . As shown, the LIDAR system includes alaser source 1201, a first beam splitter 1203, and a first polarizingbeam splitter 1205. In this example embodiment, a single laser source1201 is split using the first beam splitter 1203, and then recombinedusing the first polarizing beam splitter 1205 to generate a beam ofco-propagating, cross-polarized light. The light can then be amplifiedusing an optical amplifier 1209, and directed to a target 1217 throughan optical path discriminator 1211. The target path light can bedirected to the target 1217 through a lens system 1213 and apolarization wave plate 1215. In some embodiments, the polarization waveplate 1215 (or some other element within the system) can reflect aportion of the light that is slightly offset, or ahead of, the lightthat exits the system, enters the target environment, and is reflectedfrom the target 1217. This internal reflection can be created by anywaveguide-to-air interface (including a fiber-to-air interface), or acalibrated reflected optic in the path (including a lens, window,retroreflector, or partial mirror). Both the light reflected from thepolarization wave plate 1215 and the light reflected from the target canbe directed through the optical path discriminator 1211 to a secondpolarizing beam splitter 1219.

According to some embodiments, the optical path discriminator 1211 canbe a circulator or a beam splitter. The polarizing beam splitter 1219then directs the light to detectors 1223 and 1227, which are incommunication with a signal processing unit 1250 configured to analyzesignals from the detectors. In such an embodiment, a mixer, such as themixers 221, 225 discussed above in reference to FIG. 2 , is notrequired. The signal processing unit 1250 can compare the intensities ofthe signals from the first detector 1223 and the second detector 1227and provide insights into the target material reflectivity, surfacequality, or orientation. According to some embodiments, a variablepolarization rotator can also be included within the system in order totransform the polarization state of the beam of polarized light, asdiscussed herein.

FIG. 13 illustrates flow chart of an example method 1300 for performingmaterial estimation using polarized light in accordance with exampleimplementations of the present disclosure. In embodiments, variousportions of method 1300 may be performed by LIDAR systems of FIGS. 1, 2,and 4-12 . With reference to FIG. 13 , method 1300 illustrates examplefunctions used by various embodiments. Although specific function blocks(“blocks”) are disclosed in method 1300, such blocks are examples. Thatis, embodiments are well suited to performing various other blocks orvariations of the blocks recited in method 1300. It is appreciated thatthe blocks in method 1300 may be performed in an order different thanpresented, and that not all of the blocks in method 1300 may beperformed.

At block 1301, a beam of co-propagating, cross-polarized light isgenerated. In some embodiments, the co-propagating, cross-polarizedlight is generated using a polarizing beam splitter and a single lasersource, while in other embodiments multiple laser sources can be used.In some embodiments, the multiple laser beams may have differentwavelengths. The generated beam may be transmitted to a target, andreflected light from the target can be detected at a number ofdetectors.

At block 1303, a material characteristic or orientation of the target isdetermined using the co-propagating, cross-polarized light. In oneexample embodiment, different polarizations of the cross-polarized lightmay be reflected from the target based on certain characteristics of thetarget. For example, an ice patch on a road may largely reflect lightonly in one polarization parallel to the horizon, such that there is alarge amount of light detected polarized parallel to the horizon but notmuch light detected polarized perpendicular to the horizon. If this isthe case, the reflectivity or orientation of the target i.e. the patchof ice, can be determined. A similar phenomenon can also occur withwindows on a building that are highly reflective in one polarization.

FIG. 14 depicts a flow diagram of another method 1400 for performingmaterial estimation using polarized light in accordance with exampleimplementations of the present disclosure. In embodiments, variousportions of method 1400 may be performed by LIDAR systems of FIGS. 1, 2,and 4-12 . With reference to FIG. 14 , method 1400 illustrates examplefunctions used by various embodiments. Although specific function blocks(“blocks”) are disclosed in method 1400, such blocks are examples. Thatis, embodiments are well suited to performing various other blocks orvariations of the blocks recited in method 1400. It is appreciated thatthe blocks in method 1400 may be performed in an order different thanpresented, and that not all of the blocks in method 1400 may beperformed.

At block 1401, a beam of co-propagating, cross-polarized light isgenerated. In some embodiments, the co-propagating, cross-polarizedlight is generated using a polarizing beam splitter and a single lasersource, while in other embodiments multiple laser sources can be used.In some embodiments, the multiple laser beams may have differentwavelengths. The generated beam may be transmitted to a target, andreflected light from the target can be detected at a number ofdetectors. In some embodiments, two or more laser sources can be used togenerate a beam with different frequency patterns. In other embodiments,a WDM may be used to combine light with different wavelengths from twoor more laser sources.

At block 1403, the beam of co-propagating, cross-polarized light issplit into a LO path and a target path. This can be done, for example,using a beam splitter. At block 1405, the target path light is directedto the target using an optical path discriminator, and light reflectedfrom the target is directed to a second polarizing beam splitter. Atblock 1407, the second polarizing beam splitter splits light reflectedfrom the target and directs it to at least two mixers. At block 1409,the LO path light is split into two portions and also directed to themixers.

At block 1411 the outputs from the second polarizing beam splitter andthe third polarizing beam splitter are mixed using the mixers. In someembodiments, where a single laser source is used to generate the beam ofco-propagating, cross-polarized light, the reflected outputs from thesecond and third polarizing beam splitters are combined using a firstmixer, and the transmitted outputs from the second and third polarizingbeam splitters are combined using a second mixer. In other embodiments,where two or more laser sources are used to generate the beam ofco-propagating, cross-polarized light, the transmitted output from thesecond polarizing beam splitter is combined with the reflected output ofthe third polarizing beam splitter at the first mixer, while thereflected output from the second polarizing beam splitter is combinedwith the transmitted output of the third polarizing beam splitter at thesecond mixer. The first and second mixers are in communication with afirst and second detectors, and combined light from the first mixer isreceived at the first detector and combined light from the second mixeris received at the second detector.

At block 1413, a material characteristic or orientation of the target isdetermined based on a comparison of a SNR between signals from a firstand second detector. As discussed above, in some cases the mixers canbias light to the detectors in favor of the target path. As discussedabove, a comparison of the SNR between signals from the detectors canindicate the reflectivity or orientation of the target.

FIG. 15 depicts a flow diagram of a method for performing specklereduction using polarized light in accordance with exampleimplementations of the present disclosure. In embodiments, variousportions of method 1500 may be performed by LIDAR systems of FIGS. 1,and 8-12 . With reference to FIG. 15 , method 1500 illustrates examplefunctions used by various embodiments. Although specific function blocks(“blocks”) are disclosed in method 1500, such blocks are examples. Thatis, embodiments are well suited to performing various other blocks orvariations of the blocks recited in method 1500. It is appreciated thatthe blocks in method 1500 may be performed in an order different thanpresented, and that not all of the blocks in method 1500 may beperformed.

At block 1501, a beam of polarized light is generated. In someembodiments, the co-propagating, cross-polarized light is generatedusing a polarizing beam splitter and a single laser source, while inother embodiments multiple laser sources can be used. In someembodiments, the multiple laser beams may have different wavelengths.The generated beam may be transmitted to a target, and reflected lightfrom the target can be detected at a number of detectors.

At block 1503, the polarization state of the polarized beam istransformed using a variable polarization rotator at a rate faster thanthe data collection rate from the detectors. As discussed above, rapidlymodulating the polarization state of the signal transmitted to thetarget can average the spatial-mode coherence, thereby reducing thenegative effects of speckle.

FIG. 16 depicts a flow diagram of another method for performing specklereduction using polarized light in accordance with exampleimplementations of the present disclosure. In embodiments, variousportions of method 1600 may be performed by LIDAR systems of FIGS. 1,and 8-12 . With reference to FIG. 16 , method 1600 illustrates examplefunctions used by various embodiments. Although specific function blocks(“blocks”) are disclosed in method 1600, such blocks are examples. Thatis, embodiments are well suited to performing various other blocks orvariations of the blocks recited in method 1600. It is appreciated thatthe blocks in method 1600 may be performed in an order different thanpresented, and that not all of the blocks in method 1600 may beperformed.

At block 1601, a beam of polarized light is generated. In someembodiments, the co-propagating, cross-polarized light is generatedusing a polarizing beam splitter and a single laser source, while inother embodiments multiple laser sources can be used. In someembodiments, the multiple laser beams may have different wavelengths.The generated beam may be transmitted to a target, and reflected lightfrom the target can be detected at a number of detectors.

At block 1603, the beam of co-propagating, cross-polarized light issplit into a LO path and a target path. This can be done, for example,using a beam splitter. At block 1605, the target path light is directedto the target using an optical path discriminator, and light reflectedfrom the target is directed to a first polarizing beam splitter. Atblock 1607, the first polarizing beam splitter splits light reflectedfrom the target and directs it to at least two mixers. At block 1609,the LO path light is split into two portions and also directed to themixers. In embodiments where the VPR is located ahead of the target pathand LO path splitter, the LO path is split into two portions using asecond polarizing beam splitter. Where the VPR is located after the LOpath splitter, then a standard 2×2 splitter can be used to split the LOpath and direct it to the mixers.

At block 1611, the first and second light mixers mix light from thefirst polarizing beam splitter and the second polarizing beam splitter(or the second 2×2 splitter). The first and second mixers are incommunication with first and second detectors, and combined light fromthe first mixer is received at the first detector and combined lightfrom the second mixer is received at the second detector. As discussedabove, in some cases the mixers can bias light to the detectors in favorof the target path.

At block 1613, the polarization state of the polarized beam istransformed using a variable polarization rotator at a rate faster thanthe data collection rate from the detectors. By rapidly modulating thepolarization state of the signal transmitted to the target, one canaverage the spatial-mode coherence and reduce the negative effects ofspeckle.

FIG. 17 illustrates a diagrammatic representation of a machine in theexample form of a computer system 1700 within which a set ofinstructions, for causing the machine to perform any one or more of themethodologies discussed herein, may be executed. In alternativeembodiments, the machine may be connected (e.g., networked) to othermachines in a local area network (LAN), an intranet, an extranet, or theInternet. The machine may operate in the capacity of a server or aclient machine in a client-server network environment, or as a peermachine in a peer-to-peer (or distributed) network environment. Themachine may be a personal computer (PC), a tablet PC, a set-top box(STB), a Personal Digital Assistant (PDA), a cellular telephone, a webappliance, a server, a network router, a switch or bridge, a hub, anaccess point, a network access control device, or any machine capable ofexecuting a set of instructions (sequential or otherwise) that specifyactions to be taken by that machine. Further, while only a singlemachine is illustrated, the term “machine” shall also be taken toinclude any collection of machines that individually or jointly executea set (or multiple sets) of instructions to perform any one or more ofthe methodologies discussed herein.

The exemplary computer system 1700 includes a processing device 1702, amain memory 1704 (e.g., read-only memory (ROM), flash memory, dynamicrandom-access memory (DRAM), a static memory 1706 (e.g., flash memory,static random access memory (SRAM), etc.), and a data storage device1718, which communicate with each other via a bus 1730. Any of thesignals provided over various buses described herein may be timemultiplexed with other signals and provided over one or more commonbuses. Additionally, the interconnection between circuit components orblocks may be shown as buses or as single signal lines. Each of thebuses may alternatively be one or more single signal lines and each ofthe single signal lines may alternatively be buses.

Processing device 1702 represents one or more general-purpose processingdevices such as a microprocessor, central processing unit, or the like.More particularly, the processing device may be complex instruction setcomputing (CISC) microprocessor, reduced instruction set computer (RISC)microprocessor, very long instruction word (VLIW) microprocessor, orprocessor implementing other instruction sets, or processorsimplementing a combination of instruction sets. Processing device 1702may also be one or more special-purpose processing devices such as anapplication specific integrated circuit (ASIC), a field programmablegate array (FPGA), a digital signal processor (DSP), network processor,or the like. The processing device 1702 is configured to executeprocessing logic 1726, which may be one example of the signal processingunit 250 of FIG. 2 or the polarization rotating controller 850 of FIG. 8, for performing the operations and steps discussed herein.

The data storage device 1718 may include a machine-readable storagemedium 1728, on which is stored one or more set of instructions 1722(e.g., software) embodying any one or more of the methodologies offunctions described herein, including instructions to cause theprocessing device 1702 to execute the signal processing unit 250 or thepolarization rotating controller 850. The instructions 1722 may alsoreside, completely or at least partially, within the main memory 1704 orwithin the processing device 1702 during execution thereof by thecomputer system 1700; the main memory 1704 and the processing device1702 also constituting machine-readable storage media. The instructions1722 may further be transmitted or received over a network 1720 via thenetwork interface device 1708.

The machine-readable storage medium 1728 may also be used to storeinstructions to perform the signal processing unit 250 of FIG. 2 or thepolarization rotating controller 850 of FIG. 8 , as described herein.While the machine-readable storage medium 1728 is shown in an exemplaryembodiment to be a single medium, the term “machine-readable storagemedium” should be taken to include a single medium or multiple media(e.g., a centralized or distributed database, or associated caches andservers) that store the one or more sets of instructions. Amachine-readable medium includes any mechanism for storing informationin a form (e.g., software, processing application) readable by a machine(e.g., a computer). The machine-readable medium may include, but is notlimited to, magnetic storage medium (e.g., floppy diskette); opticalstorage medium (e.g., CD-ROM); magneto-optical storage medium; read-onlymemory (ROM); random-access memory (RAM); erasable programmable memory(e.g., EPROM and EEPROM); flash memory; or another type of mediumsuitable for storing electronic instructions.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present disclosure. Thus, the specific details set forth are merelyexemplary. Particular embodiments may vary from these exemplary detailsand still be contemplated to be within the scope of the presentdisclosure.

Additionally, some embodiments may be practiced in distributed computingenvironments where the machine-readable medium is stored on and orexecuted by more than one computer system. In addition, the informationtransferred between computer systems may either be pulled or pushedacross the communication medium connecting the computer systems.Embodiments of the claimed subject matter include, but are not limitedto, various operations described herein. These operations may beperformed by hardware components, software, firmware, or a combinationthereof.

The preceding description sets forth numerous specific details such asexamples of specific systems, components, methods, and so forth, inorder to provide a good understanding of several embodiments of thepresent disclosure. It will be apparent to one skilled in the art,however, that at least some embodiments of the present disclosure may bepracticed without these specific details. In other instances, well-knowncomponents or methods are not described in detail or are presented insimple block diagram format in order to avoid unnecessarily obscuringthe present disclosure. Thus, the specific details set forth are merelyexemplary. Particular embodiments may vary from these exemplary detailsand still be contemplated to be within the scope of the presentdisclosure.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure, orcharacteristic described in connection with the embodiments included inat least one embodiment. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment” in various places throughout thisspecification are not necessarily all referring to the same embodiment.In addition, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or.”

Although the operations of the methods herein are shown and described ina particular order, the order of the operations of each method may bealtered so that certain operations may be performed in an inverse orderor so that certain operation may be performed, at least in part,concurrently with other operations. In another embodiment, instructionsor sub-operations of distinct operations may be in an intermittent oralternating manner.

The above description of illustrated implementations of the invention,including what is described in the Abstract, is not intended to beexhaustive or to limit the invention to the precise forms disclosed.While specific implementations of, and examples for, the invention aredescribed herein for illustrative purposes, various equivalentmodifications are possible within the scope of the invention, as thoseskilled in the relevant art will recognize. The words “example” or“exemplary” are used herein to mean serving as an example, instance, orillustration. Any aspect or design described herein as “example” or“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Rather, use of the words“example” or “exemplary” is intended to present concepts in a concretefashion. As used in this application, the term “or” is intended to meanan inclusive “or” rather than an exclusive “or”. That is, unlessspecified otherwise, or clear from context, “X includes A or B” isintended to mean any of the natural inclusive permutations. That is, ifX includes A; X includes B; or X includes both A and B, then “X includesA or B” is satisfied under any of the foregoing instances. In addition,the articles “a” and “an” as used in this application and the appendedclaims should generally be construed to mean “one or more” unlessspecified otherwise or clear from context to be directed to a singularform. Moreover, use of the term “an embodiment” or “one embodiment” or“an implementation” or “one implementation” throughout is not intendedto mean the same embodiment or implementation unless described as such.Furthermore, the terms “first,” “second,” “third,” “fourth,” etc. asused herein are meant as labels to distinguish among different elementsand may not necessarily have an ordinal meaning according to theirnumerical designation.

What is claimed is:
 1. A method of operating a frequency-modulatedcontinuous wave (FMCW) light detection and ranging (LIDAR) systemcomprising: transmitting a beam of co-propagating, cross-polarized lightto a target; receiving return light reflected from the target, whereinthe return light comprises a first polarization and a secondpolarization; splitting the return light reflected from the target intoa first output comprising the first polarization and a second outputcomprising the second polarization using a first beam splitter, whereinthe first output is directed to a first detector and the second outputis directed to a second detector; generating, by the first detector, afirst electrical signal corresponding to the first polarization;generating, by the second detector, a second electrical signalcorresponding to the second polarization; determining an orientation ofthe target based on the first electrical signal and the secondelectrical signal; and generating a point cloud based on the orientationof the target.
 2. The method of claim 1, wherein the first polarizationis parallel to a horizon corresponding to the FMCW LIDAR system, andwherein the second polarization is perpendicular to the horizon.
 3. Themethod of claim 2, wherein the determining further comprises: responsiveto determining that a first signal-to-noise ratio of the firstelectrical signal is larger than a second signal-to-noise ratio of thesecond electrical signal, determining that the target is on a surfacetraveled upon by the FMCW LIDAR system.
 4. The method of claim 2,wherein the determining further comprises: responsive to determiningthat a first signal-to-noise ratio of the first electrical signal isless than a second signal-to-noise ratio of the second electricalsignal, determining that the target corresponds to a reflectivestructure perpendicular to a surface traveled upon by the FMCW LIDARsystem.
 5. The method of claim 1, further comprising: splitting the beamof co-propagating, cross-polarized light into a local oscillator (LO)path beam and a target path beam using a second beam splitter, whereinthe target path beam is transmitted to the target; splitting the LO pathbeam into a first LO polarization output and a second LO polarizationoutput using a third beam splitter; mixing the first LO polarizationoutput with the first output comprising the first polarization using afirst optical mixer to produce a first mixed optical signal; and mixingthe second LO polarization output with the second output comprising thesecond polarization using a second optical mixer to produce a secondmixed optical signal.
 6. The method of claim 5, further comprising:biasing light to the first detector and the second detector in favor oflight from the target path beam using the first optical mixer and thesecond optical mixer.
 7. The method of claim 5, further comprising:receiving light from a plurality of optical sources at the second beamsplitter.
 8. The method of claim 5, further comprising: combining afirst pair of light inputs having dissimilar wavelengths and oppositepolarizations using a first wavelength division multiplexer (WDM);combining a second pair of light inputs having dissimilar wavelengthsand opposite polarizations using a second WDM; receiving combined lightfrom the first WDM and the second WDM at the first beam splitter; andseparating the combined light from the first optical mixer and thesecond optical mixer by wavelength before directing the first output andthe second output to the first detector and the second detector.
 9. Themethod of claim 5, further comprising: inputting the first mixed opticalsignal into the first detector to produce the first electrical signal;and inputting the second mixed optical signal into the second detectorto produce the second electrical signal.
 10. A frequency-modulatedcontinuous wave (FMCW) light detection and ranging (LIDAR) apparatuscomprising: an optical transmitter to transmit a beam of co-propagating,cross-polarized light to a target; an optical receiver to receive returnlight reflected from the target, wherein the return light comprises afirst polarization and a second polarization; a first beam splitter tosplit the return light reflected from the target into a first output anda second output, wherein the first output comprises the firstpolarization and is directed to a first detector, and wherein the secondoutput comprises the second polarization and is directed to a seconddetector; the first detector to receive the first output and produce afirst electrical signal corresponding to the first polarization; thesecond detector to receive the second output and produce a secondelectrical signal corresponding to the second polarization; and aprocessing device configured to: determine an orientation of the targetbased on the first electrical signal and the second electrical signal;and produce a point cloud based on the orientation of the target. 11.The apparatus of claim 10, wherein the first polarization is parallel toa horizon corresponding to the FMCW LIDAR system, and wherein the secondpolarization is perpendicular to the horizon.
 12. The apparatus of claim11, wherein the processing device is configured to: determine that thetarget is on a surface traveled upon by the FMCW LIDAR system inresponse to a first signal-to-noise ratio of the first electrical signalbeing larger than a second signal-to-noise ratio of the secondelectrical signal.
 13. The apparatus of claim 11, wherein the processingdevice is configured to: determine that the target corresponds to areflective structure perpendicular to a surface traveled upon by theFMCW LIDAR system in response to a first signal-to-noise ratio of thefirst electrical signal being less than a second signal-to-noise ratioof the second electrical signal.
 14. The apparatus of claim 10, furthercomprising: a second beam splitter to split the beam of co-propagating,cross-polarized light into a local oscillator (LO) path beam and atarget path beam; a third beam splitter to split the LO path beam into afirst LO polarization output and a second LO polarization output; afirst optical mixer to mix the first LO polarization output with thefirst output comprising the first polarization to produce a first mixedoptical signal; and a second optical mixer to mix the second LOpolarization output with the second output comprising the secondpolarization to produce a second mixed optical signal.
 15. The apparatusof claim 14, wherein the first optical mixer and the second opticalmixer are configured to bias light to the first detector and the seconddetector in favor of light from the target path beam.
 16. The apparatusof claim 10, further comprising: a second beam splitter configured toreceive light at a first light input and a second light input andtransmit the beam of co-propagating, cross-polarized light to thetarget, wherein the first light input and the second light input areconfigured to receive light from a plurality of light sources.
 17. Theapparatus of claim 16, wherein the plurality of light sources isconfigured to transmit light having different frequency patterns. 18.The apparatus of claim 16, further comprising a wavelength divisionmultiplexer (WDM) configured to combine light from a first light sourceand a second light source when the first light source and the secondlight source have dissimilar wavelengths.
 19. A frequency-modulatedcontinuous wave (FMCW) light detection and ranging (LIDAR) systemcomprising: a plurality of optical circuits to generate and receive alaser beam, the plurality of optical circuits comprising: an opticalreceiver to receive return light reflected from a target, wherein thereturn light comprises a first polarization and a second polarization; afirst beam splitter to split the return light reflected from the targetinto a first output and a second output, wherein the first outputcomprises the first polarization and is directed to a first detector,and wherein the second output comprises the second polarization and isdirected to a second detector; the first detector to receive the firstoutput and produce a first electrical signal corresponding to the firstpolarization; a second detector to receive the second output and producea second electrical signal corresponding to the second polarization; anda processing device configured to: determine an orientation of thetarget based on the first electrical signal and the second electricalsignal; and produce a point cloud based on the orientation of thetarget.
 20. The system of claim 19, the processing device configured to:determine that the target is on a surface traveled upon by the FMCWLIDAR system in response to a first signal-to-noise ratio of the firstelectrical signal being larger than a second signal-to-noise ratio ofthe second electrical signal; and determine that the target correspondsto a reflective structure perpendicular to a surface traveled upon bythe FMCW LIDAR system in response to the first signal-to-noise ratio ofthe first electrical signal being less than the second signal-to-noiseratio of the second electrical signal.